To cleave hydrocarbons, a stream of hydrocarbon is passed through a cracking tube or coil and thermally cracked at high temperatures and pressures within the coils. The coils can be made of high-alloy austenitic (including Fe—Ni—Cr alloy) stainless steel pipes joined through welding. In some instances, a CrNi weld material can be used.
Despite being made of high quality alloy austenitic steel, the coils can undergo progressive embrittlement or corrosion from the inside and on the outside. On the outside, the exterior of the coils can come into contact with heating gas (including natural gas or high methane natural gas) used in the cracking furnace as well as gaseous combustion products contained in the exhaust stream (including CO2, CO, and NOx). It is believed that such gases can diffuse into the coils because of the high surface temperatures and allow for the formation of nitrides and carbo-nitrides, thereby causing embrittlement of the coil material and reduce the lifespan of the coils. In some instances, from the inside and because of thermal stress on the coil material, carbon atoms can diffuse via the wetted interior surface of the coils into the coil material, thereby contributing to embrittlement of coil material. In some instances, coil materials are damaged during operation by carburizing, nitriding, internal oxidation, depletion of chromium, and stress cracking due to longitudinal elongation.
Under the influence of high temperatures and pressures, such corrosion or embrittlement can lead to cracks or tears in the coil walls, which in turn, can lead to coil rupture and failure of the cracking furnace.
To prevent coil ruptures or furnace failures, the coils can be replaced at regular intervals. However due to the high costs involved in replacing a coil in a cracking furnace, an ideal time for replacing a coil is when the corrosion or embrittlement of the coil material has reached a state that the coil is considered to have reached “end of life”. For this reason, coils are inspected at regular intervals. Notably, metallurgical and destructive testing require dismantling coils and thus are not appropriate for inspecting coils in situ. Non-destructive visual inspections including dimension analyses of the coil (including, for example. determination of changes in dimensions or form), while helpful in detecting certain potential failure indicators, the visual inspections do not allow for recognition of damage due to chromium depletion, nitriding, or carburizing.